I'm pretty sure I've experienced this as well, and the pressure must be pretty high to do that sort of thing "accidentally". It's basically free money for them.
Is it even illegal though? Would it need to be 'accidental' or could they just make it an internal policy?
It doesn't have to be a policy. Large beaurocracies naturally tend toward certain behaviors due to natural incentives. You must have policies/regulations to specifically counteract these tendencies.
Seems like the kind of thing that is probably illegal if they're overt enough, but I don't know. It's definitely not ethical or moral or admirable in any way.